US intelligence reports written before the Iraq war warned President George Bush that an invasion could lead to an insurgency, the New York Times reports.
Hundreds of Iraqis have died in militant suicide attacks
The reports also predicted the war would increase sympathy in the Islamic world for some terrorist objectives, officials who saw the reports say.
The classified reports were written in early 2003 by the National Intelligence Council, which advises the CIA.
A White House spokesman said Mr Bush had known the potential dangers.
"The president was very well aware of the challenges that we faced if the decision was made to go and remove Saddam Hussein from power," Scott McClellan was quoted as saying by Reuters news agency.
"He's also very well aware of the consequences of not
acting to remove Saddam Hussein's regime and hold them accountable in a post-September 11 world," he added.
Three government officials who had seen or been briefed on the assessments told the New York Times that the documents said it was unlikely that Iraq would split apart.
However, the report warned that there could be a guerrilla warfare insurgency against the new Iraqi government or US-led forces.
There was a significant chance that domestic groups would engage in violent internal conflict with one another, it added.
One of the documents also described the building of democracy in Iraq as a long, difficult and potentially turbulent process, with the possibility that Iraq could slide into authoritarianism.
Earlier this month, US officials acknowledged the existence of another pessimistic report on Iraq by the National Intelligence Council.
It put forward three possible scenarios for the country's future, ranging from what it called "tenuous stability" to political fragmentation and civil war.